Today we will implement a tcp router or tcp portforwarder as it works against only 1 endpoint.

let opts = ForwardOptions(listenAddr:"127.0.0.1", listenPort:11000.Port, toAddr:"127.0.0.1", toPort:6379.Port)
var f = newForwarder(opts)
asyncCheck f.serve()
runForever()

and then you can do

redis-client -p 11000
> PING
PONG

• Listen on listenPort on address listenAddr and accept connections.
• On every new connection (incoming)
  • open a socket to toPort on toAddr (outgoing)
  • whenever data is ready on any of both ends write the data to the other one

Linux provides APIs like select, poll to watch or monitor set of file descriptors and allows you to do some action on whatever ready file descriptor for reading or writing.

The select() function gives you a way to simultaneously check multiple sockets to see if they have data waiting to be recv()d, or if you can send() data to them without blocking, or if some exception has occurred.

Please check Beej's guide to network programming for more on that

import  strformat, tables, json, strutils, sequtils, hashes, net, asyncdispatch, asyncnet, os, strutils, parseutils, deques, options, net

Options for the server specifying on which address to listen and where to forward the traffic.

type ForwardOptions = object
  listenAddr*: string
  listenPort*: Port
  toAddr*: string
  toPort*: Port

type Forwarder = object of RootObj
  options*: ForwardOptions


proc newForwarder(opts: ForwardOptions): ref Forwarder =
  result = new(Forwarder)
  result.options = opts

Represents the server the forwarder

and newForwarder creates a forwader and sets its options

proc serve(this: ref Forwarder) {.async.} =
  var server = newAsyncSocket(buffered=false)
  server.setSockOpt(OptReuseAddr, true)
  server.bindAddr(this.options.listenPort, this.options.listenAddr)
  echo fmt"Started tcp server... {this.options.listenAddr}:{this.options.listenPort} "
  server.listen()
  
  while true:
    let client = await server.accept()
    echo "..Got connection "

    asyncCheck this.processClient(client)

We will utilize async/await features of nim to build our server.

• Create a new socket with newAsyncSocket (make sure to set buffered to false so Nim doesn't try to read all requested data)

• setSockOpts allows you to make the socket reusable

SO_REUSEADDR is used in servers mainly because it's common that you need to restart the server for the sake of trying or changing configurations (some use SIGHUP to update the configuration as a pattern) and if there were active connections the next time you start the server will fail.

• bindAddr binds the server to certian address and port listenAddr and listenPort
• then we start a loop to recieve connections.
• we should call await processClient right? why asyncCheck processClient

• await means execute that async action and block the execution until you get a result.
• asyncCheck means execute async action and don't block a suitable name might be discard or discardAsync

No we can answer the question why call asyncCheck processClient instead of await processClient is because we will block the event machine until processClient completely executes which defeats the purpose of concurrency and accepting/handling multiple clients.

proc processClient(this: ref Forwarder, client: AsyncSocket) {.async.} =
  let remote = newAsyncSocket(buffered=false)
  await remote.connect(this.options.toAddr, this.options.toPort)
  ...

First thing is to get a socket to the endpoint where we forward the traffic defined in the ForwardOptions toAddr and toPort

No we could've established a loop and reading data from the client socket and write it to the remote socket

Problem is we may get out of sync, sometimes the remote sends data once a client connects to it before reading anything from the client. Maybe the remote sends information like server version or some metadata or instructions on protocol and it may not we can't be sure that it's waiting on recieving data always as the first step. So what we can do is watch the file descriptors and whoever has data we write to the other one.

e.g

• remote has data: we read recv it and write send it to the client.
• client has data: we read recv it and write send it to the remote.

  proc remoteHasData() {.async.} =
    while not remote.isClosed and not client.isClosed:
      echo " in remote has data loop"
      let data = await remote.recv(1024)
      echo "got data: " & data
      await client.send(data)
    client.close()
    remote.close()

  proc clientHasData() {.async.} =
    while not client.isClosed and not remote.isClosed:
      echo "in client has data loop"
      let data = await client.recv(1024)
      echo "got data: " & data
      await remote.send(data)
    client.close()
    remote.close()

Now let's register clientHasData and remoteHasData procs to the event machine and LET'S NOT BLOCK on any of them (remember if you don't want to block then you need asyncCheck)

  try:
    asyncCheck clientHasData()
    asyncCheck remoteHasData()
  except:
    echo getCurrentExceptionMsg()

So now our processClient should look like

proc processClient(this: ref Forwarder, client: AsyncSocket) {.async.} =
  let remote = newAsyncSocket(buffered=false)
  await remote.connect(this.options.toAddr, this.options.toPort)

  proc clientHasData() {.async.} =
    while not client.isClosed and not remote.isClosed:
      echo "in client has data loop"
      let data = await client.recv(1024)
      echo "got data: " & data
      await remote.send(data)
    client.close()
    remote.close()

  proc remoteHasData() {.async.} =
    while not remote.isClosed and not client.isClosed:
      echo " in remote has data loop"
      let data = await remote.recv(1024)
      echo "got data: " & data
      await client.send(data)
    client.close()
    remote.close()
  
  try:
    asyncCheck clientHasData()
    asyncCheck remoteHasData()
  except:
    echo getCurrentExceptionMsg()

let opts = ForwardOptions(listenAddr:"127.0.0.1", listenPort:11000.Port, toAddr:"127.0.0.1", toPort:6379.Port)
var f = newForwarder(opts)
asyncCheck f.serve()
runForever()

runForever begins a never ending global dispatch poll loop

our full code

# This is just an example to get you started. A typical binary package
# uses this file as the main entry point of the application.

import  strformat, tables, json, strutils, sequtils, hashes, net, asyncdispatch, asyncnet, os, strutils, parseutils, deques, options, net

type ForwardOptions = object
  listenAddr*: string
  listenPort*: Port
  toAddr*: string
  toPort*: Port

type Forwarder = object of RootObj
  options*: ForwardOptions

proc processClient(this: ref Forwarder, client: AsyncSocket) {.async.} =
  let remote = newAsyncSocket(buffered=false)
  await remote.connect(this.options.toAddr, this.options.toPort)

  proc clientHasData() {.async.} =
    while not client.isClosed and not remote.isClosed:
      echo "in client has data loop"
      let data = await client.recv(1024)
      echo "got data: " & data
      await remote.send(data)
    client.close()
    remote.close()

  proc remoteHasData() {.async.} =
    while not remote.isClosed and not remote.isClosed:
      echo " in remote has data loop"
      let data = await remote.recv(1024)
      echo "got data: " & data
      await client.send(data)
    client.close()
    remote.close()
  
  try:
    asyncCheck clientHasData()
    asyncCheck remoteHasData()
  except:
    echo getCurrentExceptionMsg()

proc serve(this: ref Forwarder) {.async.} =
  var server = newAsyncSocket(buffered=false)
  server.setSockOpt(OptReuseAddr, true)
  server.bindAddr(this.options.listenPort, this.options.listenAddr)
  echo fmt"Started tcp server... {this.options.listenAddr}:{this.options.listenPort} "
  server.listen()
  
  while true:
    let client = await server.accept()
    echo "..Got connection "

    asyncCheck this.processClient(client)

proc newForwarder(opts: ForwardOptions): ref Forwarder =
  result = new(Forwarder)
  result.options = opts

let opts = ForwardOptions(listenAddr:"127.0.0.1", listenPort:11000.Port, toAddr:"127.0.0.1", toPort:6379.Port)
var f = newForwarder(opts)
asyncCheck f.serve()
runForever()

This project is very simple, but helped us tackle multiple concepts like how to utilize async/await and asyncCheck interesting use cases (literally @dom96 explained it to me). Of course, It can be extended to support something like forwarding TLS traffic based on SNI So you can serve multiple backends (with domains) using a single Public IP :)

Please feel free to contribute by opening PR or issue on the repo.